The present invention relates generally to regenerated cellulose and more specifically to processes for preparing solutions of cellulose in a tertiary amine oxide and for producing shaped articles such as fibers and films.
The use of organic N-oxides such as tertiary amine oxides for dissolving cellulose was first reported by Graenacher and Sallman in U.S. Pat. No. 2,179,181. Subsequently, the specific use of N-Methyl Morpholine-N-Oxide (NMMO) to dissolve cellulose was disclosed by D. L. Johnson in U.S. Pat. No. 3,447,939 and U.S. Pat. No. 3,508,941. These patents disclose the use of NMMO to dissolve cellulose and the production of films and fibers by the precipitation of the dissolved cellulose.
The use of NMMO as a solvent for cellulose and the production of cellulosic fibers and films was also disclosed in McCorsley et al., U.S. Pat. No. 4,142,913 which disclose a process wherein cellulose is mixed with a tertiary amine oxide such as NMMO and a liquid non-solvent containing controlled amounts of water which assists in intimately associating the tertiary amine oxide with the fibers of the cellulose to facilitate absorption of the tertiary amine oxide. The resulting mixture is maintained at a temperature at which the non-solvent and excess water are removed so that the cellulose dissolves in the tertiary amine oxide until a solution is obtained which is suitable for shaping into a cellulosic article such as by spinning or extrusion. The non-solvent can be water or it can be a mixture of water and organic non-solvent with a boiling point below 130.degree. C. including alcohols such as n-propyl alcohol, isopropyl alcohol, butanol or an aprotic liquid such as toluene, morpholine, methyl ethyl ketone or tetrahydrofuran.
McCorsley et al., U.S. Pat. No. 4,144,080 disclose a process wherein a comminuted solid precursor of a solution of cellulose in amine oxide such as NMMO is charged to an extrusion apparatus, is heated to a temperature where the amine oxide dissolves the cellulose to form an extrudable solution of cellulose and the resulting solution is extruded through a die to form an extrudate of uniform composition. Franks et al., U.S. Pat. No. 4,145,532 disclose methods of dissolving cellulose in solutions containing water and NMMO. Turbak et al., Chemtech, p. 51-57, January, 1980 provide a review of developments in cellulose solvent systems including amine oxides. Turbak subsequently reported cellulose solutions with lithium chloride and dimethylacetamide. These references further describe the potential use of such solutions in the production of new fiber and film products.
Of specific interest to the present application is the disclosure of Johnson, U.S. Pat. No. 3,508,941 which describes the addition of various water soluble and other polymers including polymeric esters such as poly(vinyl acetate), polysaccharides such as gum arabic, and proteins such as gelatin with cellulose and uses dimethyl sulfoxide (DMSO) as an organic co-solvent for the two polymers in the presence of N-methyl-morpholine-N-oxide and other cyclic N-oxides. Nevertheless, U.S. Pat. No. 3,508,941 teaches in its examples use of at least equal amounts of the added polymer to the amount of cellulose and does not disclose use of water as a cosolvent with lowering the water concentration to a point necessary to achieve solubility of the cellulose.
The NMMO process for producing cellulosic fibers and films has become particularly attractive in recent years because of safety and environmental concerns regarding the viscose process traditionally used for production of cellulosic films and fibers. In particular, the use of carbon disulfide in the viscose system has led to a desire for a simple, more ecologically friendly closed loop totally recoverable cellulose solvent system. Use of processes for spinning cellulose from NMMO solutions continues with the manufacture of over 120 million lbs/year of cellulose fibers via the NMMO process in about 1996.
While the NMMO system for production of cellulosic films and fibers provides various benefits over use of the viscose system it is also subject to certain limitations. This is particularly the case with respect to the ability to control the precipitation of the cellulose. In the viscose system, the cellulose is first made into a xanthate derivative by the use of caustic soda and carbon disulfide. This derivative is then spun into a coagulation/regeneration bath containing high salt and low to medium acid so that the cellulose can first be congealed into a gelatinous mass and densified by the salt in a controlled manner. This allows the xanthate solubilizing groups on the cellulose molecules sufficient opportunity to permit alignment and packing of the cellulose into the proper positions to make a good quality product. The longer the regeneration is retarded, the higher is the resulting product quality since more effective stretching and alignment can be obtained in the densifying coagulated system.
In contrast to methods of using the viscose system, the cellulose molecules in tertiary amine oxide processes are not derivatized but are directly dissolved by the action of the tertiary amine oxide. This dissolving takes place over very narrow limits of water content. As may be seen from the graphs published by Franks et al. in U.S. Pat. No. 4,145,532 and also by Chanzy et al. "Swelling and Dissolution of Cellulose in Amine Oxide/Water Systems," Ninth Cellulose Conference, State University of New York, Syracuse, N.Y., May 24-27, 1982) there are relatively narrow concentration ranges for dissolving and maintaining cellulose in solution. Beyond these ranges, for example, further addition of water causes very rapid and drastic precipitation of the cellulose out of solution. See also, Turbak, TAPPI Journal, Vol. 67., No. 1 pp. 94-96 (1984). Thus, as compared to the viscose process, the ability to control the coagulation and precipitation of cellulose in a tertiary amine oxide process is substantially diminished thus hindering the ability to provide for orderly spinning of a fiber or extrusion of a film. While such rapid and dramatic precipitation is advantageous from the view of getting high spinning speeds, it is disadvantageous with respect to being able to control and improve the nature of the product properties. Accordingly, there exists a need in the art for methods to retard and control the rapid precipitation of the cellulose from NMMO and other tertiary amine oxide solutions in order that better control of molecular structure can be obtained during the coagulation and precipitation of the cellulose molecules.